GHSA-pq4g-8rxf-4g29

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GHSA-pq4g-8rxf-4g29

Summary: Crass: Incorrect tokenization of bad URLs enables CSS sanitizer bypass

Details: ## Summary Crass's tokenizer didn't terminate a bad URL token at a `)` that followed an escaped backslash (`\\`). As a result, Crass consumed more input as part of the bad URL than a spec-compliant browser does. CSS sanitizers that parse untrusted CSS with Crass and re-emit it could be tricked into passing through declarations, style rules, or at-rules (including `@import`) that the sanitizer would otherwise have removed, but that a browser would then parse and apply. ## Details While consuming the remnants of a bad URL (CSS Syntax Level 3, §4.3.15), Crass checked whether the *previously consumed* code point and the next code point formed a valid escape, in addition to checking the next two unconsumed code points. After an escaped backslash, the previously consumed code point is a backslash, so a following `)` was incorrectly treated as escaped and the bad URL token continued until the next unescaped `)` or the end of the input. Browsers terminate the bad URL at that `)`, so everything Crass treated as part of the bad URL is live CSS to a browser. This bug has existed in every released version of Crass. ## Impact Crass by itself only mis-tokenizes input. The security impact is on CSS sanitizers that use Crass to make allow/deny decisions and then re-serialize the CSS. Exploitation requires a sanitizer that doesn't reject bad URL tokens wherever they occur. Confirmed examples (verified in Chromium against the sanitizers' output): - **Sanitize** (all versions through 7.0.0): - In `Sanitize::CSS.properties` and `style` attributes, a bad URL inside a `[...]` or `(...)` block in the value of an allowlisted property can smuggle arbitrary declarations, such as `position: fixed` or a `background-image` URL with a disallowed protocol. - In `Sanitize::CSS.stylesheet` and `<style>` elements, a bad URL in a selector can smuggle arbitrary style rules and at-rules, including `@import`. - **Loofah** (`Loofah::HTML5::Scrub.scrub_css`, used for `style` attributes), and therefore **rails-html-sanitizer** (`sanitize_css`, and `sanitize` when `style` is a permitted attribute): a bad URL inside an allowlisted function such as `calc()` or `rgb()` can smuggle declarations Loofah normally strips, such as `position: fixed` and `url()` values. Smuggled CSS can be used for UI redress (clickjacking or phishing overlays), loading attacker-controlled external resources, and, where rules or `@import` can be smuggled, CSS-based data exfiltration. This vulnerability doesn't allow script execution in modern browsers. ## Patches Fixed in Crass 1.0.8. Users of Sanitize, Loofah, rails-html-sanitizer, or any other library that uses Crass to sanitize CSS should upgrade Crass to 1.0.8 or later. ## Workarounds There are no practical workarounds other than upgrading. Rejecting any CSS input that contains the character sequence `\\` would block the known trigger, but this is a blunt measure and may reject legitimate CSS. ## References - CSS Syntax Module Level 3, "Consume the remnants of a bad url": https://drafts.csswg.org/css-syntax-3/#consume-remnants-of-bad-url

References: https://github.com/rgrove/crass/security/advisories/GHSA-pq4g-8rxf-4g29, https://nvd.nist.gov/vuln/detail/CVE-2026-116695, https://github.com/rgrove/crass/commit/689fc45c632fdf3803b2f4e844ffc9cca3b08f34, https://github.com/rgrove/crass, https://github.com/rgrove/crass/releases/tag/v1.0.8, https://github.com/rubysec/ruby-advisory-db/blob/master/gems/crass/GHSA-pq4g-8rxf-4g29.yml